2. OBJECTIVES
At the end of the lesson, the students will be able to:
1. Discuss dextrotatory and levorotatory designations of carbohydrates;
2. Differentiate hemiacetal from hemiketal formation; and
3. List down sugar derivatives of Carbohydrates
3. OVERVIEW
Carbohydrates have the following basic composition:
I
(CH2O)n or H - C - OH
I
Monosaccharides - simple sugars with multiple OH groups. Based on
number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose,
pentose or hexose.
Disaccharides - 2 monosaccharides covalently linked.
Oligosaccharides - a few monosaccharides covalently linked.
Polysaccharides - polymers consisting of chains of monosaccharide or
disaccharide units.
4. MONOSACCHARIDES
Aldoses (e.g., glucose) have an
aldehyde group at one end.
Ketoses (e.g., fructose) have
a keto group, usually at C2.
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
5. D VS L DESIGNATION
D & L designations are based on the
configuration about the single
asymmetric C in glyceraldehyde.
The lower representations are Fischer
Projections. CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-glyceraldehyde
6. SUGAR NOMENCLATURE
For sugars with more
than one chiral center, D
or L refers to the
asymmetric C farthest
from the aldehyde or
keto group.
Most naturally occurring
sugars are D isomers.
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
7. D & L sugars are mirror
images of one another.
They have the same
name, e.g., D-glucose
& L-glucose.
Other stereoisomers
have unique names,
e.g., glucose, mannose,
galactose, etc.
The number of stereoisomers is 2n, where n is the
number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thus there
are 16 stereoisomers (8 D-sugars and 8 L-sugars).
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
8. HEMIACETAL & HEMIKETAL FORMATION
An aldehyde can react with an
alcohol to form a hemiacetal.
A ketone can react with an
alcohol to form a hemiketal.
O C
H
R
OH
O C
R
R'
OHC
R
R'
O
aldehyde alcohol hemiacetal
ketone alcohol hemiketal
C
H
R
O R'R' OH
"R OH "R
+
+
9. Pentoses and hexoses can
cyclize as the ketone or aldehyde
reacts with a distal OH.
Glucose forms an intra-
molecular hemiacetal, as the C1
aldehyde & C5 OH react, to form
a 6-member pyranose ring, named
after pyran.
These representations of the cyclic sugars are called
Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
23
4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose
(linear form)
10. Fructose forms either
a 6-member pyranose ring, by reaction of the C2 keto group with the OH
on C6, or
a 5-member furanose ring, by reaction of the C2 keto group with the OH
on C5.
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
H
OH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
